The present invention relates to a stitching head, which can be used in a machine for producing a reinforcement in the form of a sheet for a composite component formed by said reinforcement embedded in a cured matrix, said reinforcement including superposed plies of thread (especially carbon, glass or boron thread) and to a machine comprising such a stitching head.
More particularly, although not exclusively, such composite components are used in the aeronautical and space fields by virtue of their excellent properties of resistance to mechanical and/or thermal stresses.
Many methods and machines are known for obtaining said composite components and, especially, their reinforcement. In general, the first step is to produce the reinforcement from fibers (threads), in particular inorganic fibers, distributed in at least two directions, after which the material of the matrix in injected into the reinforcement and said material is cured in order to form said matrix and, thus, said composite component.
A description is given, for example in the Applicant's Patent EP-B-0,284,497, of a method and a machine for producing a reinforcement for a composite component formed by said reinforcement embedded in a cured matrix, said reinforcement including superposed plies of thread, according to which method:
at least some of said plies are produced by laying down straight portions of thread, forming part of a continuous thread, on a support, pulling tight each thread portion between two points for positionally fixing the ends of said portion, the ply thus consisting of portions of thread extending at least substantially in a parallel fashion with respect to each other, and the portions of thread of said ply extending in a direction which is either parallel or crossed with respect to the direction of the portions of thread of any other ply in the reinforcement; and PA1 all the superposed plies are consolidated by stitching, without knotting, a continuous thread passing through said plies.
In this case, in order to lay down the straight portions of thread constituting the corresponding plies, a special device is used which comprises a tube, open at its lower end and having an opening at its upper part, for the passage of a continuous thread. This laying-down head allows each thread portion to be pulled tight between points for positionally fixing its ends, these fixing points being formed by pins temporarily implanted in the support made of a material which can be penetrated by said pins, such as a foam of synthetic material. After forming the superposed plies of thread and after consolidating said plies, the pins are removed from the support.
As already indicated, the consolidation of all the superposed plies is carried out by stitching, without knotting, a continuous thread passing through said plies, this being achieved by using a stitching head, mounted on said machine, comprising a support carrying, on the one hand, means of linkage to the machine and, on the other hand, a needle as well as means for guiding the consolidating thread right to said needle.
However, in Patent EP-B-0,284,497, this involves producing at least substantially cylindrical reinforcements and, in order to do this, the support is in the form of a likewise cylindrical mandrel. Thus, each ply consisting of straight portions of thread is already compressed by the superjacent ply consisting of a length of continuous thread, wound around said mandrel, which superjacent ply also makes it possible, as required, to compress and compact all the subjacent plies already laid down on the mandrel. The consolidation of the superposed plies by stitching, without knotting, a continuous thread passing through said plies may therefore be performed, as illustrated by FIGS. 10a to 10f of Patent EP-B-0,284,497, simply by inserting the thread through the plies using the needle, the thread being retained therein in the form of loops because of the very fact that the plies through which it passes are already, by construction, compressed. The consolidating thread reinforces the compaction and compression effect, obtained beforehand when laying down the plies consisting of circumferential lengths of thread, and ensures that all the plies are held together and that the reinforcement has mechanical integrity.
However, when it is desired to produce reinforcements in the form of sheets (plane or curved) from superposed plies each consisting of straight portions of thread, the problem of consolidating the superposed plies is of a different nature.
In fact, in this case, if possible interlacing of the successive plies is disregarded, which is not desirable either for other reasons, the superposed plies are, by construction, not compressed. One solution for solving this problem is mentioned in Patent EP-0,056,351. Here, the reinforcement consists of plies called "warp thread" plies and of plies called "filling thread" plies, these being superposed in parallel planes, the threads of the plies not being mutually interlaced, and the upper ply and the lower ply of the set of plies consisting of "filling" plies. Consolidation of said "warp" and "filling" plies is achieved by means of binding threads which pass through these plies, going around the filling threads of the external (upper and lower) plies so as to form, together with them, a typical fabric trapping the internal warp and filling plies.
It should be noticed that, in this case, the compaction of the warp and filling plies is solely achieved by tensioning the binding threads. Indeed, as they are arranged, the binding threads require, obviously, a shuttle in order to lay them down, which necessitates the plies having a low "density" of the warp and filling threads (or a large spacing between these threads), as well as a small thickness, this consequently requiring a tension to be exerted on the binding threads in order to hold in place and to compact the warp and filling threads. Such a tension runs the risk, of course, of breaking the binding threads which are by nature fragile. Moreover, the solution indicated hereinabove finally results in producing, at least in part, a typical fabric by interlacing the filling threads of the external plies and the binding threads, these being in fact equivalent to warp threads.